The use of PMMA bone cements is based on the pioneering work of Sir Charnley (Charnley, J.: Anchorage of the femoral head prosthesis of the shaft of the femur. J. Bone Joint Surg. 42 (1960) 28-30.). The structure of PMMA bone cements has remained basically the same ever since. PMMA bone cements consist of a liquid monomer component and a powder component. The monomer component generally contains the monomer, methylmethacrylate, and an activator (N,N-dimethyl-p-toluidine) dissolved therein. The powder component consists of one or more polymers that are made by polymerisation, preferably suspension polymerisation, based on methylmethacrylate and co-monomers, such as styrene, methylacrylate or similar monomers, a radio-opaquer, and the initiator, dibenzoylperoxide. Mixing the powder component and the monomer component, a dough that can be shaped plastically is generated by swelling of the polymers of the powder component in the methylmethacrylate. In this context, the activator, N,N-dimethyl-p-toluidine, reacts with dibenzoylperoxide forming radicals in the process. The radicals thus formed trigger the radical polymerisation of the methylmethacrylate. Upon advancing polymerisation of the methylmethacrylate, the viscosity of the cement dough increases until the cement dough solidifies.
Polymethylmethacrylate bone cements can be mixed by mixing the cement powder and the monomer liquid in suitable mixing beakers with the aid of spatulas. This procedure is disadvantageous in that air inclusions may be present in the cement dough thus formed and cause destabilisation of the bone cement later on. For this reason, it is preferred to mix the bone cement powder and the monomer liquid in vacuum mixing systems. Mixing in a vacuum allows air inclusion to largely be removed from the cement dough and optimal cement quality to thus be attained (Breusch S. J. et al.: Der Stand der Zementiertechnik in Deutschland. Z Orthop. 1999, 137: 101-07). Bone cements mixed in a vacuum have clearly reduced porosity and thus show improved mechanical properties. Various vacuum cementing systems have been described of which only the following shall be named for exemplary purposes: U.S. Pat. No. 6,033,105, U.S. Pat. No. 5,624,184, U.S. Pat. No. 4,671,263, U.S. Pat. No. 4,973,168, U.S. Pat. No. 5,100,241, W99/67015, EP1020167, U.S. Pat. No. 5,586,821, EP1016452, DE3640279, WO94/26403, EP0692229, EP1005901, U.S. Pat. No. 558,745, U.S. Pat. No. 5,344,232.
Aside from removing air or gas inclusions through mixing in vacuo, DE430223005 proposes in addition to also use a vacuum to collect the polymethylmethacrylate bone cement that has been mixed in a vacuum. Said system largely prevents air inclusions in the cement which arise when the polymethylmethacrylate bone cement dough collect at the head end of the cartridges and the remaining residual air between the cement dough and the plunger at the cartridge head is expelled together with the polymethylmethacrylate bone cement dough through the dispensing dispensing tube.
Another concept for attaining a polymethylmethacrylate bone cement that is largely free of air inclusions is used in the Palamix® vacuum cementing system made by Heraeus Medical GmbH, Hanau, that is currently commercially available. In this cementing system, a gas-permeable pore disc is situated in the cartridge head. The air that is present can escape through said pore disc when the polymethylmethacrylate bone cement dough is being dispensed. How-ever, said concept cannot be used with vacuum cementing systems, in which the pore disc is integrated into the plunger.
Aside from the polymethylmethacrylate bone cement that are produced based on powder and liquid, cements made from pastes are also known. According to the invention, the term, polymethylmethacrylate bone cement, includes all production forms of cements, in particular polymethylmethacrylate bone cements made from pastes aside from the cements made from powder and liquid, such as are described, e.g., in DE 102007050762 B3.
It is therefore the object of the invention to provide a simple and safe device and a method for dispensing polymethylmethacrylate bone cements that allow a polymethylmethacrylate bone cement dough that is largely free of air inclusions to be produced.